Field of the Invention
The invention relates generally to solubilizing polymers and, in particular, to reversible derivatization of poly (aryl ether ketones) (PAEKs) to enhance their solubility.
Description of the Related Art
Over the past 20 years, a new class of high performance materials, poly (aryl ether ketones), or PAEKs, have emerged and gained commercial importance. PAEKs having a wide range of ether/ketone ratios have been developed in order to tailor the properties of the resulting materials. Examples of commercially important PAEKs include poly (ether ketone) (PEK), poly (ether ether ketone) (PEEK), poly (ether ketone ketone) (PEKK) and poly (ether ketone ether ketone ketone) (PEKEKK). These materials are thermoplastics which possess high glass transition temperatures, greater than about 140° C., good stability to oxidation, and low dielectric constants. They also substantially retain these mechanical properties at elevated temperatures.
PAEKs have been employed for the fabrication of medical instruments which are in direct contact with the body, such as endoscopes, cannula, and cardiac pump systems. These semi-crystalline materials show significant solvent resistance and chemical stability over a wide range of temperatures and are commonly used as injection loop stators and fittings in chromatography systems such as high performance liquid chromatography (HPLC) and gel permeation chromatography (GPC), also known as high-pressure size exclusion chromatography. PAEKs are further resistant to many chemicals widely used in the aerospace industry, including dichloromethane (CH2Cl2), lubricating oils, hydraulic fluids, and gasoline, and are thus used in housing to protect electric wires and fiber optic filaments in aircraft production. The low flammability and low smoke emissions of PAEKs also makes them excellent candidates for commercial aircraft interiors. Furthermore, polymer composites based on PAEKs have excellent mechanical characteristics and can be very light. For example, Aromatic Polymer Composite-2 (APC-2, Cytec, Inc.) is a composite possessing a PEEK matrix with reinforcing carbon fiber that is now used extensively in the construction of both commercial and military aircraft.
While the resistance of PAEKs to chemicals is often a benefit in their commercial utility, this property also limits the ability to characterize PAEKs. For example, a high performance liquid chromatography (HPLC) technique such as gel permeation chromatography (GPC), separates components based on their molecular size in solution to determine the molecular weight distribution of a polymer. A PAEK such as PEEK, however, can absorb organic solvents such as dichloromethane, o-dichlorobenzene, or N,N-dimethylformamide (DMF), giving rise to solvent-induced crystallization and plasticization. Furthermore, common organic solvents have little or no effect on semi-crystalline PEEK, even at elevated temperatures. Thus, little information is known about the mass distribution of these materials because of the difficulty in finding a PAEK solvent compatible with GPC.
The commercial importance of PEEK and other PAEKs has thus attracted significant attention to the problem of PAEK solubility and a variety of methods have been developed for characterizing and solubilizing PAEKs. In one approach, PAEKs have been characterized by the inherent viscosity of the dilute polymer solution in concentrated sulfuric acid. Unfortunately, this technique does not provide a measure of the polydispersity index, a measure of the distribution of molecular weights in the polymer. In another approach, it has been found that a mixture of phenol and 1,2,4-trichlorobenzene can be used as an eluent for PEEK GPC analysis at about 115° C. However, these solvents are highly toxic and, therefore, unsuitable for routine characterization of PEEK or other PAEKs.
In another example, PEEK may be sulfonated according to the reaction:
The sulfonated PEEK has been found to be soluble in dipolar aprotic solvents such as DMF and N-Methylpyrrolidone (NMP).
In a further example, the addition of nitric acid to a solution of PEEK in methanesulfonic acid (MSA) leads to the formation of nitro-derivatives of PEEK.

A significant deficiency with the routes illustrated above, however, is that each route works with different efficiency. That is to say, relatively few PAEKs are capable of sulfonation, even under harsh conditions. For example, sulfonation works on PAEKs such as PEEK but not on polymers such as PEK. Thus, a general route which works efficiently for all PAEKs is not known.
Another drawback of these routes is that the polymer chains often experience irreversible chemical changes or degradation, yielding a final polymer with different properties from the starting polymer. For example, in the formation of nitro-derivatives of PEEK, it is believed that the polymer chains are degraded because of the strong decrease observed in the solution viscosity of the nitrated PEEK. Thus, this derivative is not suitable for GPC analysis.
One method to produce soluble PAEKs is the polymerization of monomers that will impart solubility to the polymer. This would involve selecting a monomer that has, eg bulky side groups to disrupt the crystallinity of the polymer formed, and polymerizing this to make a soluble PAEK. This method is useful for preparing soluble PAEKs but does not allow the use of standard commercial PAEKs. This method also necessitates a selected monomer and polymerisation for each soluble PAEK that is desired. This method is therefore not general in the sense that standard commercial PAEKs cannot be made soluble after they have been polymerized but must be made soluble at the time of polymerization.
From the foregoing, there is a need for improved methods of solubilizing PAEK systems. In particular, there is a need for a method of solubilizing PAEKs which is broadly applicable to PAEKs and is substantially reversible.